1. Field of the Invention
This invention relates, in general, to frequencystabilizer oscillators, more particularly, to dielectricresonator-stabilized oscillators (DRSO's), and most specifically, to an improved form of a dielectric resonator (DR) to be used in conjunction with a GalliumArsenide (GaAs) Field Effect Transistor (FET) oscillator operating in the microwave region which interconnects in a tuned circuit to the DR by means of a transmission line in the form of a slab-type microstripline, as well as a method for tuning the same to a precise, desired frequency on a production basis by the use of a trimming device such as a laser-trimmer.
2. Summary of the Prior Art
Electrical oscillators are used widely in the electronics industry for a variety of purposes. Oscillators operating within the microwave region have come to be commonly utilized in the fields of radar and telecommunications, in which the oscillators are employed as sub-components in circuits having timing or distance-measuring functions, for example.
It is well known in the art that a negative resistance oscillator can be created by adding the proper choice of terminating impedance to an unstable, active element.
At frequencies in the microwave region, it is common to employ a GaAs FET as the active element of the oscillator circuit in order to take advantage of the inherent instability of these devices. Typical of oscillator circuits incorporating these kinds of devices as an active element are those illustrated in FIGS. 2, 3, and 4 in which the configurations illustrated are for a common drain, a common source and a common gate oscillator circuit, respectively.
An additional requirement placed upon oscillators, particularly those operating in timing and measuring applications, is that of frequency stability. It is well known in the art that, in a given oscillator circuit of the type described above, the resonant frequency of the circuit will be subject to both long-term and short-term variations, due primarily to variations in electrical parameters of the elements of the circuit with time, temperature, etc., as well as the random effect of stray signals coupled into the circuit. For this reason, it is not uncommon to incorporate into the oscillator circuit a resonant device which is used to determine the frequency at which the circuit will oscillate. Moreover, it is known that, if the elements of the resonator device are selected such that they introduce a large variation of phase with frequency, then the oscillator frequency will tend to depend more completely only upon the properties of the resonator itself and will become nearly independent of all other variations in circuit parameters. Thus, a tuned-circuit oscillator can be made to have excellent frequency stability, provided that the Q of the resonator is sufficiently high and that the reactances in the resonator are relatively stable.
A well known, analogous tuned-circuit feedback oscillator which has widespread use in the industry is that which incorporates a piezo-electric crystal, usually quartz, in which the electro-mechanical system formed thereby provides oscillators having resonant frequencies ranging from a few kilohertz to a few megahertz which are extremely stable with respect to time and temperature.
In the 1-20 GHz region, a different mechanization is called for, and it is not uncommon to employ a resonator having a resonant cavity filled with a dielectric material electrically coupled into a negative resistance oscillator circuit. Typically, this dielectric material is air, but evacuated resonant cavities have also been employed. One objection presented by these tuned cavities is their size, in that the dielectric constant of a vacuum or air is relatively low, necessitating a larger cavity to achieve the appropriate reactance of the resonator. Recently, due primarily to the development of dielectric materials whose physical properties and stability with time and temperatures can be more precisely controlled, it has become possible to implement a tuned-circuit oscillator which employs a resonant cavity containing a resonator body of dielectric material which, when coupled effectively into the oscillator circuit, is capable of producing frequency stabilities on the order of 5 ppm/.degree.C. in that frequency range with resonators having significantly smaller dimensions. To couple the DR into the oscillator circuit, a transmission line technique may be employed. An additional degree of stability can be achieved (to 1ppm/.degree.C.) by the incorporation of a second DR circuit in the output leg of the oscillator.
The initial resonant frequency of the circuit is achieved by careful control of the dimensions and materials of the DR and its initial placement along the transmission line to achieve the proper phase relationship with respect to the output of the active circuit. The oscillator is then finely-tuned by the means of minute adjustments of the dimensions of the DR cavity.
Typically, this fine-tuning is accomplished by providing the DR cavity with a movable wall, i.e., a wall comprising a threaded tuning screw which is adjusted upward or downward to adjust the dimensions of the resonating cavity. Obviously, the mating threads between the tuning screw and the walls of the DR cavity must be precisely formed, finely pitched and have little or no effective backlash in order to permit precision tuning of the device. Moreover, a mechanism for fixing the tuning screw in position, once the desired frequency has been achieved, and without affecting the frequency previously tuned, must be incorporated. Typically, this is accomplished by means of a locking nut or by careful bonding of the tuning screw in place.